C3P NEW SPREAD OF CASSAVA BROWN STREAK VIRUS DISEASE AND ITS Crop .

C3PCropCrisisControlProjectNEW SPREAD OF CASSAVA BROWNSTREAK VIRUS DISEASE AND ITSIMPLICATIONS FOR THE MOVEMENTOF CASSAVA GERMPLASM IN THEEAST AND CENTRAL AFRICAN REGIONPheneas Ntawuruhunga and James LeggBrief 3BackgroundThe East Africa Root Crops Research Network (EARRNET), a networkof the Association for Strengthening Agricultural Research in East andCentral Africa (ASARECA), and executed by the International Instituteof Tropical Agriculture (IITA), wishes to bring to the attention of allstakeholders interested in the promotion of cassava in East and CentralAfrica that new reports of spread of cassava brown streak disease (CBSD)suggest that this disease now represents a significantly increased threatto the region’s cassava.New SpreadPrior to 2004, CBSD had never been recorded at high incidence above1000 meters above sea level, and was primarily known as a disease ofthe lowland cassava-growing areas of East Africa, including the shores ofLake Malawi. However, from late 2004 onwards it became apparent thatCBSD was becoming more and more widespread in parts of south-centralUganda. Preliminary survey work confirmed the identity of the virusaffecting cassava in Uganda to be the ipomovirus, Cassava brown streakvirus, CBSV (Alicai et al. 2007).Dr. Pheneas Ntawuruhunga: International Institute of Tropical Agriculture-Uganda & EasternAfrica Root Crops Research Network.Dr. James Legg: International Institute of Tropical Agriculture-Tanzania, PO Box 6226, Dar esSalaam, Tanzania.

NEW SPREAD OF CASSAVA BROWN STREAK VIRUS DISEASE AND ITS IMPLICATIONS FORTHE MOVEMENT OF CASSAVA GERMPLASM IN THE EAST AND CENTRAL AFRICAN REGIONSymptoms of cassava brown streak disease on lowerleaves of a susceptible cassava variety – Kibandameno(Kenya). Characteristics are an irregular yellow blotchychlorosis that is most pronounced on lower leaves.Chlorosis is often also associated with secondary andtertiary veins. In severe cases there may be leaf dryingand shoot die‑back.Symptoms of cassava brown streak disease in the rootsof a susceptible cassava variety (Tanzania). The diseasecauses a dry corky rot that is most pronounced in theperiphery of the root cortex. Symptoms range from paleyellow flecks to extensive areas of dark brown dry rot andbecome more severe as the plant ages.Cassava brown streak disease symptoms in roots of thesusceptible variety, TME 204 (Namulonge, Uganda). Rootconstrictions and malformation are a common feature ofsevere CBSD.Cassava brown streak disease leaf symptoms in TME 14(Namulonge, Uganda). This image clearly demonstratesthe association of chlorosis with secondary and tertiaryveins.

NEW SPREAD OF CASSAVA BROWN STREAK VIRUS DISEASE AND ITS IMPLICATIONS FORTHE MOVEMENT OF CASSAVA GERMPLASM IN THE EAST AND CENTRAL AFRICAN REGIONAn important feature of CBSD in Uganda is thatincidence is highest and severity greatest inCMD‑resistant varieties that are being promoted forthe management of the CMD pandemic. This is mostprominent for the two varieties TME 14 and TME 204that have proved to be highly popular with farmers,and have therefore spread very rapidly within andbetween farming communities. Importantly, however,CBSD symptoms can also now be seen in diverselocal cultivars, although in this case symptoms appearto be less severe. It also seems likely that this ‘new’spread of the virus causing CBSD is being enhancedby the super-abundance of the whitefly vector, Bemisiatabaci, a phenomenon that has been associated withthe CMD pandemic. This suggests that all parts of theregion already affected by the CMD pandemic withconcomitant super-abundant whitefly populations maybe particularly vulnerable to CBSD spread.than hitherto realized, with numerous occurrencesreported from local cultivars using CBSV diagnostictests (C. Herron, personal communication). In westernKenya, a significant ‘outbreak’ has also been reportedfrom a large multiplication site in the Yala Swamparea of Western Province. Virtually all plants of theCMD-resistant variety MM96/5280 were infected. Insouthwestern Democratic Republic of Congo (DRC),the disease continues to spread and root symptomswere reported at M’vuazi in regional trials on thevarieties MM 96/0287, MM 96/7204, MM 96/7762,Nsansi, and RAV. Significantly, the last two varieties inthis list are the most widely-grown released varietiesin the DRC. These observations highlight the regionalthreat posed by CBSD and emphasize the importanceof having clearly-defined strategies for managementboth within countries and at the regional level.IITA and Uganda’s National Agricultural ResearchOrganization conducted a survey of cassava pestsand diseases in January 2007 which incorporatedan assessment of CBSD leaf symptoms for all of the493 fields sampled through 26 of the most importantcassava-growing districts in the country. CBSD wasrecorded from 40 of the fields, distributed virtuallythroughout the country, but with greatest incidencein south, central areas (Figure 1), corresponding tothe area in which cultivation of TME and TME-likevarieties is most widespread. Although almost 10% ofall fields had CBSD infected plants, overall incidencewas relatively low at 1.9%. This was likely to be anunderestimate since cassava brown streak virus(CBSV) may be latent (i.e. some infected plants may besymptomless) and some varieties express symptomsin roots but not in leaves. In order to provide a moreauthoritative estimate for CBSV incidence it would benecessary to combine assessments of both shoots androots with tests of both symptomatic and asymptomaticplant tissues using virus diagnostics. Although suchdiagnostic protocols are available, they are not widelyused, and the only African laboratories currently activein detecting and diagnosing CBSV are the MikocheniAgricultural Research Institute (MARI)-IITA group inDar es Salaam and the National Agricultural ResearchOrganisation (NARO) in Uganda.A Natural Resources Institute (NRI, UK) review missioncarried out in 2005 concluded that whilst it was nolonger possible to eradicate CBSD in Uganda, aneradication approach should be feasible for neighboringcountries and regions where similar conditions pertainsuch as western Kenya, northwestern Tanzania,Rwanda, Burundi and eastern DRC. A first step,however, should be the survey of all introducedmaterials to determine whether or not CBSD is present,particularly in the varieties most commonly affected inUganda: TME 204 and TME 14.In addition to these observations from Uganda,recent survey work in northwestern Tanzania hasshown that CBSV may be much more commonControl approachesThe CBSD threat has clear implications for germplasmexchange within the region. Recognizing theimportance of this situation, EARRNET stakeholdersrequested the Network’s coordination team to developa statement addressed to the Committee of Directorsof ASARECA on the appearance of the disease in theregion and call for common action from all stakeholdersto restrict the movement of cassava cuttings.As a part of this communication, it was proposed thatonly virus-tested tissue culture materials be used forinter-country cassava germplasm movements. Notably,the most recent introductions of germplasm to bothRwanda and Burundi, from the Kenya Plant HealthInspectorate Service (KEPHIS) Plant Quarantine Stationat Muguga, Nairobi, and supported by EARRNET, havebeen in tissue culture form.Much remains to be done, however. Diagnosticcapabilities remain virtually at the level at which they

NEW SPREAD OF CASSAVA BROWN STREAK VIRUS DISEASE AND ITS IMPLICATIONS FORTHE MOVEMENT OF CASSAVA GERMPLASM IN THE EAST AND CENTRAL AFRICAN REGIONCBSD incidenceNot surveyedNo CBSD 0- 3 %3 - 10 % 10 ueZambiaFigure 1. Recent CBSD emergence in the Great Lakes Region. The Red zone is the area recognized as the primarynew CBSD-affected area. Red spots indicate new reports of CBSD. Recent spread of CBSD in Uganda hasoccurred since 2004. CBSD is endemic to coastal zones of Kenya, Tanzania and Mozambique.

NEW SPREAD OF CASSAVA BROWN STREAK VIRUS DISEASE AND ITS IMPLICATIONS FORTHE MOVEMENT OF CASSAVA GERMPLASM IN THE EAST AND CENTRAL AFRICAN REGIONwere prior to the new reports of CBSD spread, andthe only locations where CBSV diagnostics are beingroutinely run is in the laboratories of MARI, Dar esSalaam and NARO, Uganda. This is a significantlimitation, as without diagnostics, national programscientists and plant protection staff are unable todiagnose CBSV in apparently diseased samples ordetect CBSV in tissue culture plants either cominginto or going out of the country. Training activitiesconducted under C3P have provided an importantopportunity, however, to raise the level of knowledgeof the disease within the region, its symptoms andpotential methods of control. These were furthersupplemented by an EARRNET-IITA training workshopfor researchers held in Dar es Salaam in June 2007.IITA and NARS are actively involved in efforts todevelop varieties with high levels of resistance toboth CMD and CBSD. Currently, however, there arefew varieties available within C3P project target areaswith recognized sources of CBSD resistance. Asthese are being sought and incorporated into newgermplasm, interim measures will necessarily be limitedto the promotion of those varieties that show sometolerance to CBSD and the phytosanitary ‘clean-up’ ofmultiplication blocks using symptoms as the basis fordisease diagnosis.A cassava multiplication site. Cassava brown streak disease may be ‘leaf-asymptomatic’ on many varieties so thatimpact is not revealed until harvest of the roots.

NEW SPREAD OF CASSAVA BROWN STREAK VIRUS DISEASE AND ITS IMPLICATIONS FORTHE MOVEMENT OF CASSAVA GERMPLASM IN THE EAST AND CENTRAL AFRICAN REGIONBased on these observations, the following generalrecommendations can be made:Short-term Recommendations1. Assess the health status, with respect to CBSD, ofall C3P-supported multiplication blocks and provideimmediate alerts of any apparent cases of CBSD.2. Fast-track the multiplication of those varieties knownto be tolerant to CBSD (e.g. in Uganda, Nase 3 andMH96/2961).3. Over time, discourage the multiplication of thosevarieties known to be particularly susceptible toCBSD, such as TME 204, TME 14 and others (seeabove).4. In Rwanda, Burundi, western Kenya, eastern DRCand northwestern Tanzania – recommend thedestruction of any plants, in any field, showingsymptoms of CBSD.5. Tighten surveillance of border points(Rwanda‑Uganda and Tanzania borders;DRC‑Uganda border; Burundi – Rwanda andTanzania borders; Tanzania – Uganda and Kenyaborders; Kenya‑Uganda border).6. Continue to enforce the movement of cassavagermplasm between countries only in virus-testedtissue culture form.7. Extend training to all extension staff on the cause,symptoms and control of CBSD.Longer-term Research NeedsMuch of the current difficulty being faced in CBSVcontrol stems from the very poor understanding ofwhat the virus is, how it causes disease in plants andhow it spreads. Substantially strengthened researchefforts are urgently required to help fill the major gaps inknowledge of this disease. In addition, more resourcesare required for initiatives to develop control measuresand improved disease diagnostics. Most importantamongst these will be the urgent development, throughbreeding, of resistant varieties, and there is merit inpursuing both conventional and transgenic approachesin achieving this aim.Cassava Mosic Disease (CMD). Unlike CBSD, CMD symptoms are easily identifiable making field diagnostics highlyaccurate

NEW SPREAD OF CASSAVA BROWN STREAK VIRUS DISEASE AND ITS IMPLICATIONS FORTHE MOVEMENT OF CASSAVA GERMPLASM IN THE EAST AND CENTRAL AFRICAN REGIONCassava cuttings are considered to be a primary cause of CBSD movement.